ORIGINAL ARTICLE

Energy band diagram of In: ZnO/p-Si structures deposited usingchemical spray pyrolysis technique

Marwa Abdul Muhsien Hassan • Arwaa Fadil Saleh •

Sabah J. Mezher

Received: 15 April 2013 / Accepted: 4 June 2013 / Published online: 18 June 2013

� The Author(s) 2013. This article is published with open access at Springerlink.com

Abstract Near-ideal In: ZnO/p-Si heterojunction band

edge lineup has been investigated with aid of I–V and C–V

measurements. The heterojunction was manufactured by

spray pyrolysis method of (Zn (CH3COO)2�2H2O) at dif-

ferent indium doping concentrations on monocrystalline

p-type silicon. The experimental data of the conduction

band offset DEc and valence band offset DEc were com-

pared with theoretical values. The band offset

DEc = 0.45 eV and DEv = 1.65 eV obtained at 300 K.

The energy band diagram of In: ZnO/p-Si HJ was con-

structed. C–V measurements depict that the junction was

an abrupt type and the built-in voltage was determined

from C-2–V plot.

Keywords Indium-doped ZnO � In: ZnO/p-Si

heterojunction � Electronic properties � Chemical

spray pyrolysis technique � Lineup

Introduction

Polycrystalline films have received a rapidly growing

interest due to their increasing area of applications in

advanced technologies for microelectronic, photonic, and

micromachined devices. The heterostructures for photo-

detection and photovoltaic device applications were

obtained using one of the following techniques, such as

LPE, MOCVD, etc., which are very expensive for such

applications. A good alternative low cost technology is

based on the heterojunctions with transparent conducting

oxide (TCO) thin film, such as In2O3, SnO2 have been

widely used for photovoltaic devices and recently ZnO

having a direct optical band gap of 3.45 eV, a large melting

point of 1,975 �C, a large exciton binding energy

(60 meV), and a high transparency ([80 %) for visible

light, which can be obtained with low resistivity (10-3 Xcm), is playing an important role for optoelectronic appli-

cations (Purica et al. 2000; Singh et al. 2010; Ilican et al.

2008; Zhao et al. 2006; Skriniarova et al. 2008; Major et al.

1985). Transparent conductive ZnO films can be prepared

by different methods, such as activated reactive or electron

beam evaporation, magnetron- or electron beam-sputtering,

spray pyrolysis, chemical vapor deposition with many

variants, and recently by sol–gel technique (Ilican et al.

2008; Zhao et al. 2006). The structural, physical, and

electrical properties of ZnO films were governed by

deposition parameters and post-treatment (Mondal et al.

2008; Jeong et al. 2007; Kuo and Tuan 2007). To improve

these properties can be doped with some elements. Espe-

cially, Group III elements, such as In3?, Al3?, Ga3? are

used to improve and/or control the electrical conductivity.

These dopants act as a donor when it occupies a sub-

stitutional position for Zn2? cation or an interstitial posi-

tion in the ZnO lattice. The efficiency of the dopant

element related to its electronegativity and the ionic radius.

Indium-doped ZnO films have been deposited different

methods, such as sputtering, electrodeposition process, sol–

gel deposition and spray pyrolysis. In this study, the films

have been deposited by spray pyrolysis method. This

method has variety advantages such as the low-cost, no-

vacuum and easy doping. There have been extensive

studies on the crystalline structure and optical transmit-

tance of In-doped ZnO thin films prepared by spray pyro-

lysis method. However, there are not many reports on the

study of indium dopant effect on electrical and electronical

M. A. M. Hassan (&) � A. F. Saleh � S. J. Mezher

Department of Physics, College of Science, Al-Mustansiriyah

University, Baghdad, Iraq

e-mail: marwa_alganaby@yahoo.com

123

Appl Nanosci (2014) 4:695–701

DOI 10.1007/s13204-013-0246-5

properties of In: ZnO/p-Si heterojunction device. In our

previous works, we reported on ZnO and indium-doped

ZnO films prepared by spray pyrolysis (Ilican et al. 2008).

In addition, it has been found that the n-type doping of ZnO

is relatively easy as compared to p-type doping. Group III

elements like Al, Ga and In can be used as n-type dopant.

Doping with Al, Ga and In has been attempted by many

groups, resulting in high quality, highly conducting n-type

ZnO thin films. Among group III element much of the work

has been done using Al as a dopant because the ionic radius

of Al is smaller than that of In and Ga (Singh et al. 2010;

Afify et al. 2005).The heterostructure In: ZnO/p-Si is

considered as anisotype heterojunction since In: ZnO

behaves like n-type semiconductor. The junction capaci-

tance (C) of the anisotype heterojunction of abrupt type can

be expressed as (Shama et al. 1970):

C ¼ qe1e2NAND

2 e1NA þ e2NDð Þ

� �1=2

VD � Vað Þ�1=2 ð1Þ

where e1 and e2 are permittivity of narrow band gap and

wide band gap, respectively, NA and ND are, respectively,

the free carrier concentration of p-type and n-type semi-

conductors and Va is the applied voltage.

Current transport mechanism of such heterojunction

could be explained according to any of the diffusion model,

the emission model, and the recombination model (Sharma

and Purohit 1974; Sze 1981); a relation between J and V is

represented by:

I / expqV

nKT

� �ð2Þ

where q/kT is the reciprocal of volt equivalent of temper-

ature and n is the diode factor.

The aim of this work is to obtain the electronic structure

of the In: ZnO/p-Si heterojunction and study the effect of

indium doping concentration on the electrical and elec-

tronical properties of the device.

Experimental work

Square-shaped p-type silicon samples, each of 1 9 1 cm2

area, of 1.5–4 (X cm) resistivities were prepared using a

wire-cut machine. Silicon wafers were washed ultrasoni-

cally in distilled water and were immersed in nitric acid

HNO3 for 3 min to remove ionic contamination. The

wafers were immersed in HCl: HNO3 (3:1) for 3 min to

remove metallic films. They were etched in buffered

hydrofluoric acid (34.6 % NH4F: 6.8 % HF: 58.6 % H2O)

for 2 min to remove oxide films. The silicon wafers were

cleaned in distilled water and dried in furnace at 130 �C.

The resistivity and type of conductivity of the Si substrates

were measured using 4-point probe technique. ZnO and

IZO thin films were deposited on to (111) p-type mirror-

like silicon substrates using the spray pyrolysis method.

0.2 M solution of Zn (CH3COO)2�2H2O diluted in metha-

nol and deionized water (3:1) was used for all the films. A

magnetic stirrer is incorporated for this purpose for about

10–15 min to facilitate the complete dissolution of the

solute in the solvent. Organic solvents are preferable over

distilled water because the former enables the attainment of

homogeneous, highly transparent, thin films of small grain

size. For indium doping, InCl3 was added to starting

solution. The In/Zn ratio in the solution was 0, 2, 4 and 6 %

(These films were named as ZnO, IZO2 and IZO4,

respectively). The details of spray pyrolysis set-up are

given in elsewhere (Singh et al. 2010). Chemical tech-

niques for the preparation of thin films have been studied

extensively because such processes facilitate the designing

of materials on a molecular level. Spray pyrolysis, one of

the chemical techniques applied to form a variety of thin

films, results in good productivity from a simple apparatus.

In the current research, zinc oxide thin films are deposited

on silicon substrates employing locally made spray pyro-

lysis deposition chamber whose main components set-up is

illustrated in the schematic diagram of Figs. (1, 2). It is

essentially made up of a precursor solution, carrier gas

Fig. 1 Schematic set-up for spray Pyrolysis technique

696 Appl Nanosci (2014) 4:695–701

123

assembly connected to a spray nozzle, and a temperature-

controlled hot plate heater.

The atomizer, illustrated in the photo plate (2-b), has an

adjustable copper capillary tube nozzle of 0–0.8 mm inner

diameter clamped to a holder and supported by a metal

tripod. The nozzle is driven by a compressed atmospheric

air. The prepared precursor solution is pumped through the

metal nozzle with a solution flow rate ranging from 1 to

2 mL/min. Owing to the air pressure of the carrier gas; a

vacuum is created at the tip of the nozzle to suck the

solution from the tube after which the spray starts (Perednis

and Gauckler 2005). To regulate spraying time, a 16-Bar

Tork solenoid valve controlled by an adjustable timer has

been incorporated. The atomizer and the 1,500 W hot plate

heater are enclosed in a 1 9 1 9 1 m3 ventilation hood,

photo plate (2-a). A 220 V a.c. power was applied to the

heater and temperature was measured using a type K

(nickel–chromium) thermocouple and precision digital

temperature controller (GEMO DT109 photo plate (2-c)).

The spray rate is usually in the range 2–3 mL min-1. The

optimum carrier gas pressure for this rate of solution flow

is around 5 kg cm-2. At lower pressures, the size of the

solution droplets becomes large, which results in the

presence of recognized spots on the films and then reduc-

tion of transparency. This situation increases the scattering

of light from the surface and then reduces the transmittance

of the films.

The spray pyrolytic substrate temperature is maintained

within 450 ± 5 �C during the deposition. Film thickness is

controlled by both the precursor concentration and the

number of sprays, or alternatively, spraying time. Thus, a

4 s spray time is maintained during the experiment. The

normalized distance between the spray nozzle and substrate

was fixed at 30 cm. The film thicknesses were found to be

approximately 150 nm. The silicon sample was used as

substrate for TCO’s/Si heterojunction. Ohmic contacts

were fabricated by evaporating 99.999 purity aluminum

wires for back and front contact using Edwards coating

system, after contact and assembly processes I–V charac-

teristic under different operating temperatures for In: ZnO/

p-Si sample. C–V characteristics of the produced hetero-

junction were measured using a PM6306 programmable

LRC meter supplied by Fluke at 1 MHz and reverse bias

voltage ranged from 0.2 to 2.5 V. The cross point (1/

C2 = 0) of the (1/C2–V) curve represents the built-in

potential (Vbi) of the heterojunction (Sze 1981), the

depletion layer width has been estimated using the fol-

lowing equation:

Fig. 2 Photo plate.

a Experimental set-up of the

spray pyrolysis deposition SPD.

b Air atomizer and c Gemo

DT109 temperature controller

Appl Nanosci (2014) 4:695–701 697

123

w ¼ffiffiffiffiffiffiffiffiffiffiffiffi2esVD

qNd

sð3Þ

The energy band diagram of In: ZnO/p-Si heterojunction

was constructed theoretically depending on the

experimental result, which is used in the analysis of the

capacitance–voltage characteristics, photocurrent spectra,

current voltage characteristics of photo- and dark

conductivity and their temperature dependences. The

Fermi level energy has been found using the following

equation (Sppaval and Herman 1995):

Ec � Efn ¼ KT

qln

Nc

Nd

� �

Efp � EV ¼ KT

qln

NV

Na

� �9>>>=>>>;

ð5Þ

The difference between the two conduction band

energies is denoted by DEC and the difference between

the two valance band energies is denoted by DEV.

DEc ¼ xn � xp ð6Þ

DEv ¼ Egn� Egp

� �� xp � xn

ð6Þ

and

DEc � DEv ¼ Egn� Egp

¼ DEg ð7Þ

Eg1and Eg2

are the energy groups of narrow band and

wide gap material, respectively.

On the other hand, current mechanism employs the

emission model and its value is given in the following

equation (Sharma and Purohit 1974):

I ¼ A exp � q DEc � VDð ÞKT

� �� exp

qVa

KT

� �� 1

� �ð8Þ

where Va is the applied bias, and x1 \ x2 \ x1 ? Eg1and

u1 [ u2. x1, x2 is the electron affinity for the two semi-

conductor materials, respectively.

The following figure explains the experimental set-up:

Results and discussion

Figure 3a, b gives the C–V and 1/C2–V measurements at

different indium doping concentrations for In: ZnO/p-Si

device, respectively. The results show that the device

capacitance is inversely proportional to the bias voltage.

The reduction in the device capacitance with bias voltage

resulted from the expansion of depletion layer with the

built-in potential. The depletion layer capacitance refers to

the increment in charge per unit area to the increment

change of the applied voltage. This properly gives an

indication of the behavior of the charge transition from the

donor to the acceptor region, which was found to be

‘‘abrupt’’ which is confirmed by the relation between 1/C2

and reverse bias being a straight line. The potential barrier

at the junction can be measured by small-single capaci-

tance–volt characteristic, since band bending is primarily

on the Si side, the intercept of the curve on the x axis is

essentially equal the diffusion potential within the silicon

and it value is expected to depend on the Fermi level

position in the conduction band at high carrier concentra-

tions. The total value of the built-in voltage VD can be

calculated by extrapolating 1/C2–V plot to the point 1/

C2 = 0. The intercept voltage Vint is related to the VD by

VD ¼ Vint þ ð2kTÞ ð9Þ

where kT/q is the volt equivalent of temperature. The slope

of the straight line gives the donor concentration, which its

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

-1.5-1-0.500.511.522.53

1/C2 (nf-2/cm-4)

Reverse Voltage (Volt)

ZnO

IZO2

IZO4

IZO6

0

2

4

6

8

10

12

14

16

18

20

0 0.5 1 1.5 2 2.5 3

C (

nf/

cm2)

Reverse Voltage (Volt)

ZnO

IZO2

IZO4

IZO6

baFig. 3 a Junction capacitance

as a function of the reverse

voltage, b 1/C2 vs. reverse

voltage for (In: ZnO/p-Si)

device at different indium

doping concentration

698 Appl Nanosci (2014) 4:695–701

123

value correspond well with the known resistivity of silicon

substrate. The built in potential (Vbi), the width of the

depletion layer (W) and the bulk Fermi level of p-type

silicon substrate at different In doping concentrations are

calculated and tabulated in the following (Table 1). It is

observed the width of depletion layer (W) inversely

proportional with built in potential and the optimum

width value of the depletion layer found to be 0.79 at

2 % Indium doping concentration. A typical energy

band profile of two isolated pieces of p-and n-type

semiconductors and an equilibrium energy band profile of

an abrupt p–n heterojunction formed by bringing in

intimate contact two dissimilar semiconductors having

the different type of conductivity. The In: ZnO exhibits

n-type conductivity when it is prepared from p-type Si

substrate, which has been experimentally, measured using

4-point probe devices. It is clear that the electron affinity of

wide-band gap material (n-In: ZnO) (xn) is higher than that

of the substrate (p-Si) (xp). The formation of a

heterojunction with such forbidden gap of the two

materials completely overlaps then this case is called

staggered. In: ZnO/p-Si an isotype heterojunction,

conduction is carried out almost entirely by electrons (the

barrier to the transport of holes is much higher than the

barrier seen by electrons) and the current will be given by

the current equation of the emission model Eq. (2). The

energy band profile of In: ZnO/p-Si an isotype

heterojunction, depending on the electron affinities, work

Table 1 The obtained results from the C–V measurements

In (%) Vbi (V) Nd (cm-3) W (lm) C (nF/cm2) Ec - Ef (eV)

0 0.3 2.8 9 1015 0.34 26.02 0.23

2 0.4 0.7 9 1015 0.79 11.20 0.26

4 0.2 1.4 9 1015 0.39 22.69 0.24

6 0.25 1.0 9 1015 0.52 17.01 0.25

0

20

40

60

80

100

0 0.1 0.2 0.3 0.4 0.5 0.6

J (µ

A/c

m2)

V (Volt)

ZnO

T= 273 K

T= 271 K

T= 269 K

T= 267 K

T= 265 K

0

20

40

60

80

100

120

140

0 0.1 0.2 0.3 0.4 0.5 0.6

J ( µ

A/c

m2 )

V (Volt)

IZO2

T= 273 K

T= 271 K

T= 269 K

T= 267 K

T= 265 K

a b

0

20

40

60

80

100

120

0 0.1 0.2 0.3 0.4 0.5 0.6

J (µ

A/c

m2 )

V (Volt)

IZO6

T= 273 K

T= 271 K

T= 269 K

T= 267 K

T= 265 K0

20

40

60

80

0 0.2 0.4 0.6

J (µ

A/c

m2 )

V (Volt)

IZO4T= 273 K

T= 271 K

T= 269 K

T= 267 K

T= 265 K

c d

Fig. 4 a–c Current density at

different cooling temperatures

and at different In doping

concentrations

Appl Nanosci (2014) 4:695–701 699

123

function and energy band gaps of semiconductors. Their

necessary conditions and the current will be given by the

current equation of the emission model. Figure 4a–d gives

J–V characteristic at different cooling temperature at the

range 273–265 K the extension of each curve led to Js.

Figures 5a–d, 6 gives the Js (saturation current density) Vs

1,000/T for In: ZnO/p-Si device at different indium doping

concentrations. The decrease in J leads to decrease in Js.

The slope of this plot can give the value of the conduction

band of set DEc through Eq. (5). Neglecting interface

parameters that are different to determine, the construction

of the energy band diagram for (In: ZnO/p-Si)

heterojunction can be estimated by determining DEv

from Fig. 4c with aid of Eq. (6) depending on the value

of DEc which has been found to be 0.45 eV and hence the

value of DEv was found to be about 1.65 eV. This

approximation has been used by many workers (Sharma

and Purohit 1974). Barrier height (UB) has been calculated

for In: ZnO/p-Si device at 2 % indium doping

concentration using Eq. 12, it has been found to be

0.61 eV of the (In: ZnO/p-Si) device (Sppaval and

Herman 1995).

UB ¼ KT=q ln ½A�T2=Js� ð10Þ

where KT/q is the volt equivalent of temperature and Js is

the saturation current density.

Conclusions

Several conclusions can be drawn on the basis of obtained

experimental data as shown below:

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

3.65 3.7 3.75 3.8

J s(A

/cm

2)

1000/T K-1

ZnO

0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

2.50E-05

3.00E-05

3.65 3.7 3.75 3.8

J s(A

/cm

2)

1000/T K-1

IZO6

0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

2.50E-05

3.00E-05

3.50E-05

4.00E-05

4.50E-05

5.00E-05

3.65 3.7 3.75 3.8

J s(A

/cm

2)

1000/T K-1

IZO2

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

3.65 3.7 3.75 3.8

J s(A

/cm

2)

1000/T K-1

IZO4

a b

c d

Fig. 5 a–c Saturated current

density vs. 1,000/T and at

different In doping

concentrations

700 Appl Nanosci (2014) 4:695–701

123

• The junction formed by spray pyrolysis of Zn

(CH3COO)2�2H2O at different indium doping concen-

trations on p-type Si is an isotype.

• C–V results suggest that the junction was abrupt type.

• Experimental value of DEc seems to be consistent with

the theoretically calculated DEc with satisfactory

accuracy.

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tribution, and reproduction in any medium, provided the original

author(s) and the source are credited.

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Eg 2= 3.25eV

Eg1= 1.15eV

Ev =1.65 eV

Vacuum level

Efn= 0.12eVEfp = 0.14eV

Vbi1=0.17 eV

Vbi2=0.23eV

Ec =0.45eV

Ev

Ev

Ec

Ec

p- Si n- In: ZnO

In: ZnO

Al electrode

p- Si

Al-electrode

Fig. 6 Energy band diagram for n-In: ZnO/p-Si heterojunction

device prepared at 2 % indium doping concentration

Appl Nanosci (2014) 4:695–701 701

123